Farming is simply about xp per min. As such there are three components involved:
1 - How fast you kill mobs.
2 - How fast you can reset the mission.
3 - How much def and resistance you need to survive mobs with out slowing down.
The only way to know for sure which farming toon is best is to run with Hero stats installed. Which I have done of my main farming toons.
While any toon can farm to be the best you are really limited to Brutes and Scrappers. Blasters, Corruptors, Controlers, Dominations and so on all can farm and once you have them IOed and build they all perform descently. Four builds though in by experience excel. All of these builds have full tier 4 incarnate powers and are min maxed specifically for farming.
I ran two complete maps (SL/City map) and (Reds Fire City Map) with each to get a good settled xp rate. In each run I clocked the XP on a single lowbe (level 1 to 20) with Hero stats running. All missions were run at 54x8 with the map set for LTs only (Bosses slow the xp for any build by 100K per hour). All builds are macroed so that I always have 4 red inspirations running. Other inspirations are combined into reds when I move around.
The results were surprising.
Electric/Shield Scrapper: 500K XP per hour
Claws/Fire Brute: 275K XP per hour
Staff/Fire Brute: 250K XP per hour
SS/Fire Brute: 225K per hour
I was really surprised that Claws clearly beat SS until I looks at two items:
1 - how often the rage crash slowed things down.
2 - Claws with follow up stacks twice giving a nice constant damage buff.
In reality though none of the brutes could hold a candle to the electric / shield scrapper. Having lightning rod and shield charge up every 23 seconds or so makes it a goddess of destruction. LR+SC+FB and stuff is simply dead and you jump to the next group. By the time you get there LR has recharged or Tunderstrike is up. For the city map Shield Electric can clear it in 18 minutes compared to the brutes which took 22-23 minutes on the same map.
